1. Field of the Invention
This invention relates to an attitude angle detecting apparatus for detecting the heading and attitude of a mobile unit, such as an aircraft or a ship.
2. Description of the Prior Art
There exist conventionally used attitude angle detecting apparatus for detecting the heading and attitude of a mobile unit.
One type of such apparatus receives radio waves transmitted from a plurality of position-fixing satellites with multiple antennas located at specific positions of a mobile unit, one of the antennas being used as a reference antenna, measures phases of carrier signals of the radio waves, and determines relative positions of the antennas other than the reference antenna with respect to the reference antenna to determine the heading and attitude of a mobile unit.
Another type of such apparatus determines the attitude of a mobile unit by use of an angular velocity sensor like a rate gyro for measuring the turning rate of a mobile unit.
The aforementioned apparatus designed to determine the heading and attitude of a mobile unit from the radio waves transmitted from a plurality of position-fixing satellites makes use of important elementary technology for resolving carrier phase ambiguities.
If the radio waves from some of the position-fixing satellites are interrupted by some external obstacle, for example, it becomes impossible for the aforementioned apparatus to measure carrier phases and continue estimation of the heading and attitude (hereinafter referred to as attitude angles implying both) of the mobile unit. In such a situation, the apparatus estimates the attitude angles by using an attitude angle detecting device, such as the aforementioned angular velocity sensor. Although the apparatus resumes operation for determining carrier phase ambiguities based on the radio waves from the position-fixing satellites again when it becomes possible to receive the radio waves, it is not possible to instantly obtain the attitude angles, because it takes time to redetermine the carrier phase ambiguities.
If the length of a baseline is 3 m and the radio waves from the position-fixing satellites are interrupted for one minute, for example, it will take one to two minutes on average up to about 15 minutes at maximum to redetermine initial integer ambiguity values after the interruption.
One previous approach to the solution of this problem is to immediately determine the carrier phase ambiguities when it becomes possible to receive the radio waves again by using attitude angles estimated by an angular velocity sensor.
A conventional attitude angle detecting apparatus employing this previously known approach determines the carrier phase ambiguities upon restoring normal reception of the radio waves according to a procedure shown in a flowchart of FIG. 6.
Referring to FIG. 6, the apparatus calculates a baseline vector based on the attitude angles which have been uninterruptedly estimated by the angular velocity sensor while the radio waves have been interrupted. Next, the apparatus calculates floating ambiguities from the baseline vector and rounds off fractional parts of the floating ambiguities to obtain candidates (integer values) of a potentially true integer ambiguity. The candidates of the potentially true integer ambiguity so obtained are then examined and one candidate regarded as being correct is taken as the true integer ambiguity. According to this method, the aforementioned time needed for determining the integer ambiguity can be reduced to approximately 8 seconds on average, approximately 30 seconds at maximum.
The aforementioned conventional attitude angle detecting apparatus has a problem to be solved as described below.
The accuracy of estimated attitude angles achieved by the aforementioned conventional attitude angle detecting apparatus deteriorates when the radio waves from the position-fixing satellites are interrupted for more than a specific period of time, although this period of time varies from one minute to 5 minutes depending on various conditions. The degree of deterioration of the estimated attitude angles is considerably affected by measurement errors of the angular velocity sensor. It is likely that integer ambiguity candidates calculated from estimated values of attitude angles containing such errors do not agree with the true integer ambiguity. The integer ambiguity candidates thus calculated are more likely to disagree with the true integer ambiguity as baseline lengths (distances between multiple antennas mounted on a mobile unit) increase and as the period of interruption of the radio waves increases.
The conventional attitude angle detecting apparatus has a potential to incorrectly determine integer ambiguity candidates and provide wrong attitude angles as mentioned above. If the baseline length is 3 m and the period of interruption of the radio waves from the position-fixing satellites is one minute, for example, the probability of providing incorrect integer ambiguities could be approximately 1%.
In view of the foregoing, it is an object of the invention to provide an apparatus which can instantly calculate correct integer ambiguities even after interruption of radio waves from position-fixing satellites and quickly detect the heading and attitude of a mobile unit with high precision based on the integer ambiguities so calculated.
In one principal form of the invention, an attitude angle detecting apparatus comprises means for observing a single phase difference or a double phase difference and determining an integer ambiguity from the single or double phase difference, means for calculating the relative position of an antenna other than a reference antenna with respect to the reference antenna from the phase difference of which integer ambiguity has been determined, means for calculating attitude angles of a mobile unit and an error covariance of the attitude angles, and integer ambiguity redetermination means for calculating at least one integer ambiguity candidate from the single or double phase difference based on the attitude angles and the error covariance of the attitude angles and redetermining the integer ambiguity from the integer ambiguity candidate.
The attitude angle detecting apparatus thus constructed calculates estimated values of attitude angles, baseline vectors and floating ambiguities as well as covariances of their estimation errors, calculates a plurality of integer ambiguity candidates based on these estimated values and the covariances of their estimation errors and verifies the integer ambiguity candidates so obtained, whereby the apparatus determines a single integer ambiguity and outputs the attitude angles as shown in FIG. 7.
Since the attitude angles and the attitude angle error covariance are used in producing the integer ambiguity candidates, it is possible to redetermine the integer ambiguity from the single or double phase difference for measuring the relative positions of the antennas quickly and accurately when it is necessary to redetermine the integer ambiguity after recovery from an interruption of radio waves from position-fixing satellites of the Global Positioning System (GPS).
In another principal form of the invention, an attitude angle detecting apparatus comprises means for observing a single phase difference or a double phase difference and determining an integer ambiguity from the single or double phase difference, means for calculating the relative position of an antenna other than a reference antenna with respect to the reference antenna from the phase difference of which integer ambiguity has been determined, means for calculating attitude angles of a mobile unit and an error covariance of the attitude angles and correcting the error covariance of the attitude angles, and integer ambiguity redetermination means for calculating at least one integer ambiguity candidate from the single or double phase difference based on the attitude angles and the corrected error covariance of the attitude angles and redetermining the integer ambiguity from the integer ambiguity candidate.
The attitude angle detecting apparatus thus constructed redetermines the integer ambiguity using the attitude angles which have been obtained when it has become impossible to determine the integer ambiguity from the single or double phase difference as well as the error covariance of the attitude angles corrected by a specific correction term.
With this arrangement, it is possible to redetermine the integer ambiguity quickly and accurately under various conditions by making a specific correction to the attitude angles and the error covariance of the attitude angles.
In one feature of the invention, the attitude angle detecting apparatus uses both the aforementioned means for determining the integer ambiguity from the attitude angles and the error covariance of the attitude angles and the aforementioned means for determining the integer ambiguity from the single or double phase difference, or selectively uses one of these means, in redetermining the integer ambiguity.
With this arrangement, the apparatus redetermines the integer ambiguity in accordance with situations in which the mobile unit is placed. Specifically, the apparatus redetermines the integer ambiguity quickly and accurately by using or not using the attitude angles and the error covariance of the attitude angles when the radio waves from the position-fixing satellites are not received for a specific period of time or over depending on the duration of interruption of the radio waves, for example.
In another feature of the invention, the aforementioned means for calculating the attitude angles of the mobile unit and the error covariance of the attitude angles includes an angular velocity sensor and means for calculating the attitude angles from an output of the angular velocity sensor.
In still another feature of the invention, the aforementioned means for calculating the attitude angles of the mobile unit and the error covariance of the attitude angles includes means for predicting the attitude angles and the error covariance of the attitude angles based on an assumed model depending on conditions of the mobile unit.
By using an angular velocity sensor or an assumed model as the means for calculating the attitude angles of the mobile unit and the error covariance of the attitude angles as stated above, the apparatus can determine the integer ambiguity quickly and accurately.
In yet another feature of the invention, the integer ambiguity redetermination means includes means for calculating floating ambiguities and an error covariance of the floating ambiguities and/or means for determining the integer ambiguity candidate by the Least-squares AMBiguity Decorrelation Adjustment (LAMBDA) method.
With this arrangement, the apparatus can calculate an estimated value of the integer ambiguity and an estimated error covariance and output a plurality of integer ambiguity candidates.
These and other objects, features and advantages of the invention will become more apparent upon reading the following detailed description in conjunction with the accompanying drawings.